1. Field of the Invention
This invention relates to zoom lenses of the rear focus type and, more particularly, to zoom lenses of the rear focus type, which have as high a range as 8 and as large a relative aperture as about 2.0 in F-number, which are suited to be used in photographic cameras, video cameras or broadcast cameras.
2. Description of the Related Art
For the photographic camera or video camera, there have been previously proposed many zoom lenses which make a focusing provision in other than the front or first lens unit, or employ the so-called "rear focus" type.
In general, the rear focus type of zoom lens has an advantage over the conventional type which moves the first lens unit for purposes of focusing. For example, the effective diameter of the first lens unit becomes smaller, making it easy to minimize the size of the entire lens system. Also, close-up photography, particularly photomacrography, becomes easy to carry out. Further, because a lens unit to be moved for focusing is relatively small in size and light in weight, a weaker driving power suffices for moving the lens unit. Thus, it is made possible to speed up the focusing adjustment.
A zoom lens of such rear focus type disclosed in, for example, Japanese Laid-Open Patent Application No. Sho 63-44614, comprises, from front to rear, a first lens unit of positive refractive power, a second lens unit of negative refractive power for variation of the focal length, a third lens unit of negative refractive power for compensating for the image shift with zooming, and a fourth lens unit of positive refractive power, or is of the so-called 4-unit form, wherein the third lens unit is made use of for purposes of focusing.
However, this zoom lens, because a space for assuring the movement of the third lens unit must be created, tends to result in an increase in the physical length.
In Japanese Laid-Open Patent Application No. Sho 58-136012, the zoom section is comprised of at least three lens units, one of which is moved to effect focusing.
In Japanese Laid-Open Patent Application No. Sho 62-247316, a first lens unit of positive refractive power, a second lens unit of negative refractive power, a third lens unit of positive refractive power and a fourth lens unit of positive refractive power are arranged in this order from the object side, the second, lens unit being moved to vary the focal length, and the fourth lens unit being moved to compensate for the image shift and to perform focusing.
In Japanese Laid-Open Patent Application No. Sho 58-160913, of the four lens units, i.e., the first lens unit of positive refractive power, the second lens unit of negative power and the third and fourth lens units of positive refractive powers, the first and second lens units are made movable for varying the focal length, while simultaneously moving the fourth lens unit to compensate for the image shift. Further, one or two or more of the four lens units are moved to effect focusing.
In Japanese Laid-Open Patent Applications No. Sho 58-129404 and No. Sho 61-258217, there are shown zoom lenses each comprising, from front to rear, a first lens unit of positive refractive power, a second lens unit of negative refractive power, a third lens unit of positive refractive power, a fourth lens unit of positive refractive power and a fifth lens unit of negative refractive power, or 5-unit zoom lenses, wherein the fifth lens unit or a plurality of lens units including the fifth one are moved to effect focusing.
Japanese Laid-Open Patent Application No. Sho 60-6914 has proposed a zoom lens which has a similar form to the 5-unit zoom lens described above and which has a property that, for an object at a particular finite distance, the focusing lens unit takes a constant axial position irrespective of variation of the focal length.
Recently, in the field of video cameras, the tendency to reduce the size of a solid-state image pickup element (CCD) serving as image sensing means is advancing. In place of the conventional solid-state image pickup element of, for example, 2/3 in. or 1/2 in. size, use is made of smaller or 1/3 in. or 1/4 in. image pickup elements. Along with this has come a demand that the zoom lenses to be used be of even smaller size.
Another feature of the taking lens adapted to be used with the video camera is that the distance from the last lens surface to the image plane, that is, the back focal distance, is made so long that, as dust or foreign particles on the last lens surface cast shadow on the image receiving surface of the image pickup element, its influence to the image does not amount to an adverse effect.
However, if the design rules for the zoom lens adapted to, for example, the 1/2 in. image pickup element are applied to a zoom lens for the 1/4 in. image pickup element by reducing all the dimensions merely in proportion, the back focal distance, too, is reduced by that proportion (to 1/2). Then, the dust on the last lens surface casts its appreciable shadow onto the image receiving surface of the image pickup element, thereby giving rise to a problem that the image quality is considerably lowered.
For this reason, in view of adapting the zoom lens to be used with the video camera, the zoom lens has its back focal distance made longer than a certain value, no matter however small the size of the image pickup element may become.
In general, the use of the rear focus type in the zoom lens leads to the advantage of obtaining a minimized size of the entire lens system, enabling rapid focusing to be done, and further affording easy close-up photography.
On the other hand, however, the variation with focusing of the aberrations is caused to increase. Thus, it becomes very difficult to obtain high optical performance throughout the entire focusing range in such a manner that the size of the entire lens system is minimized.
Particularly for a large relative aperture, high-range zoom lens, the problem of maintaining the optical performance at a high level throughout the entire zooming range and throughout the entire focusing range becomes very difficult to solve.